In process automation technology, field devices are often applied for registering and/or influencing process variables. Examples of such field devices include fill level measuring devices, mass flow measuring devices, pressure- and temperature-measuring devices, pH-redox-potential measuring devices, conductivity measuring devices, etc., which, as sensors, register the process variables, fill-level, flow, pressure, temperature, pH-value and conductivity value, respectively.
Serving for influencing process variables are so-called actuators, e.g. valves, which control the flow of a liquid in a section of pipeline, or pumps, which change fill-level in a container.
A large number of such field devices are available from the firm, Endress+Hauser.
Most often, field devices are connected with superordinated units, e.g. control systems or control units. The superordinated units serve for process control, process visualizing, process monitoring.
Signal transmission between field devices and the superordinated units is frequently accomplished according to the known 4-20 mA standard by means of a 2-conductor communication connection.
If the field devices are sensors, the measured values registered by them are transmitted as electrical current signals via a signal line to the superordinated units. The measuring range of the sensors is, in such case, mapped linearly onto a 4-20 mA electrical current signal.
Often, today, sensors are not connected directly with the control unit, but, instead, via a measurement transmitter. In the measurement transmitter, there is, as a rule, galvanic isolation between the input signal delivered by the sensor and the output signal forwarded to the control unit.
Besides a 4-20 mA signal transmission, with some sensors, a digital, bidirectional communication is also possible. In process automation technology, such communication is accomplished, most often, according to the widely distributed HART-standard. In this way, sensors can be configured and parametered from a control system. Furthermore, besides the actual 4-20 mA measured value, also diagnostic information can be transmitted digitally to the control system.
Conventional measurement transmitters with galvanic signal isolation do not, however, permit pass-through of HART signals. A HART-enabled sensor cannot, therefore, when connected with a control system via such a measurement transmitter, communicate digitally with the control system.